Ignitor



Patent'ed Dec. 21, 1948 IGNITOR a William W. Rigrod, Bloomfield, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 4, 1945, serial Na. 571,256 ciaims.v (01.2504275) This invention relates to ignitors and more particularly to ignitors such as employed in ignitrons as a starting or keep-alive electrode.

The usual ignitors as heretofore made ordi-y narily contain -boron carbide, boron nitride and carbon as their principal ingredients, having superseded the more ancient ignitors of Silicon carbide and silica. w

All of these prior art ignitors are characterized by a strong negativetemperature coeflicientvof resistivity, that is to say, the ignitor resistance falls off as its temperature rises. A strong negative temperature coeflicient'of resistivity in an ignitor is undesirable, especially in welding service ignitrons, as such an ignitor tends to fire With lower voltage, and higher current when heated than when cold. Inasmuch as when a sinusoidal voltage wave is applied to an ignitor, vthe time of firing depends upon ignitor voltage, a strong dependency of ignitor voltage upon its temperature introduces wide Variations in the firing instant. Thus a large negative temperature coeflicient of resistivity reduces control of Vthe welding "heat. o

It would be an ideal condition to have the temperature coefiicient of resistivity zero, but in practice such an ideal condition is not attained although it is closely approached by the lpresent invention. There are numerous reasons why a strong negative temperature coefilcient of resistivity is undesrable in ignitrons, some of the more important of which may be briefly reviewed .as

follows:` v v 1. An ignitor having a strong negative temperature coeflicient of resistivity, and hence a high firing voltage when cold, may prevent firing for the first few cycles of voltage application. In a system intended for low duty-cycle welding serv-v ice, failure to fire for the first few cycles is disastrous. In rectifier ignitron installations, failure to fire for the first few cycles causes some tubes to fire sooner than others, and so carry an overload temporarily, resulting in possible damage to those tubes and to associated equipment 2. An ignitor having a strongnegative temperature coeficient of resistivity when used in welder-service ignitrons and when used with the firing angle phased back to a large degree in order to obtain a-low root mean square welding current, is supplied with voltage 'so Vrapidly, and is heated so little by the arc current, that it fails to -heat up appreciably and continues to run cold. This circumstance calls for a higher firlng voltage than is available at the advanced phase-back angle, and hence definitely prevents firing.

Since a strong negative temperature coeiiicient of resistivity is an inherent property of boron carbide, present-day ignitors have the foregoing undesirable characteristics and disadvantages. Furthermore, it is a fact that the firing characteristics of an ignitor are critically dependent on the properties of the insulating material constituting an ingredient of the ignitor. Present day' commonly emplo-yed insulating material for ignitors is boron nitride. Like the carbide, this nitride of boron has a strong negative coeflicient of resistance, and thus the ignitors of common usage are definitely characterized by a strong negative temperature coefiicient of resistance.

A further object of the invention is to provide an ignitor having less strong negative or more positive temperature coefiicient of resistance.

Another object of the invention is to obtain a counteraction of strong negative temperature coefiicients of resistance by introducing a materialv having a less strong negative or more positive temperature coeificient of resistance.

Still'further objects of the invention will apr-` pear as the description progresses, both by direct recitation and by implication from the context.

Referring to the accompanying drawing exemplifying the invention:

Figure 1 is a` Vertical sectional view ofan ignitron showing my improved ignitor mounted therein; and

Figures 2 to 4 are elevations of ignitors of the present invention indic'ating the novel ingredients incorporated therein.

In the specific embodiment of the invention,

and describing the exemplications of the draw-i ing in detail, the ignitor is designated thereinrby reference numeral 5, and is shown as having a tapered lower end or tip portion 6 and a larger or-more sturdy upper end or body portion 'l integrallyiformed with' the tip portion. A supporting metallic band 8 is' shown gripping the said body portlon, the band being supported by a bracket 9 in turn carried'by a lead-in Wire Ifl which is` which slight rise in firin'g voltage in cold firing-'-'''``v over voltage when hot, is an important advantage-fl derived. Moreover, beryllia is found to produce an ignitor which is far strongerf'mechanically-f than prior art ignitors madeV with* bororrnitridez Furthermore, boron nitride is soluble'i'n' water,W

whereas beryllia is insoluble and the ignitonmadef;

therewith will not decompose in the air nor even s in Water or steam, thereby assuring longer "life"` i and presenting less storage difficulties prior to sealirig'into the'ignitron. 'Itfhasalso beeni determined. thata smaller` proportionuof'beryllia than: boron nitride-is needed for an ig-nitor'to" obtainithe same cold resistance andithatlfig'nitorsf as' here .proposed using beryllia require 'lower' coldv resistance than' those using boronnitride to have* the same firing currents; Addition' of'smalll'quane' tities'of berylli'a to ignitorscontainingboronini;4 tride; or other ceramics having' the usual strong' negative' V'coefficient c'ausesV a pronounced increase inlelectrical resistance and mechanical strength'.

U'se of zirconia'as an ingredient as indicated' in' the ignitor of Fig; 3 gives'a pronounc'edJ less negative or more positivestemperature coefiicient of resistance; The'firing'voltage when'the ignitor is vhot is greater than when cold" in an ignito'r' utilizing' zirconia. This' unusual propertyV makes it possible-to-attain any desired'temperature'coeflicient by mixing zirconia in suitab'le"'propor= tionswith boron nitride or other 'ceramfcshavin'g usual' strong negative coefiicient' of resistivity:4

The benefits of the'more positive'temperature' ooelficient of resistivity'of zirconia may" beattained with the mixture of zirconia'and' beryllia* InV adopting the .proportions in the mixtures; represented by the foregoingtabulation, the per-` centages of beryllia (BeO) andlofxzirconia:(ZrOa),I weredetermined experimentally, and percentagel o f. other ingredients y.calculateol -to .give-'a totaluof 100%. Range of proportion of beryllia lies from maximum shown of 6% in the first example to the minimum of 1.6% in the fourth example. A low approximate average ofsubstantially 3% beryllia is the preferred percentage in round figures. Several percent Variation of each of the other ingredients appearsrelatively non-Critical in pomparis'on 'to the spe'cific' 'range for the b'eryllia;

v I claim:

1. An ignitor comprising ingredients of boron and carbon, and an additional ingredient of a metallic compoundinclusion whereof gives to the composition' 'ag lssi negative and more positive temperaturemoeflicient of resistance.

2;' A'n' ignitor'comprlsing ingredients of boron an'd:.carbon',-;and:am additional ingredient inclu- Vs'ionwhereof gives to the composition a less negat'ive and'more positive temperature coeificient of resistance, said additional ingredient being se- 1ected*'from-"the' group f ofI hemioalf'compounds lin which 'b'erylliuml an'd' zirconiurm are elements:

3. An ignitor' comprisin'g'l in'gredientsf of'- boron and`V carbon; and an'v additional 'ingredient inclusion where'of'fg'ivs to the*-o'ompositi'cm-` alesszneg ative'i 'andi more *positive' temperature* coeffi'ci'ent" of resistance; saidv additional ingredient'being selected from the group of chemical-*compounds consisting'of' b'eryllia andz'ireonia;4

(ZIOV 4'. ;An' ignitor comprisi'rrg in'gredients of boron and""carbon; and' an* additional'f' ingredie'ntV incl'u'-t: sionN wl'i'reof gives: to the' composition" a' less'` neg-U ative' and; more positive" temlsnerature coeifi'cient. of resistance; said? additionalJ 'ingredi'ent being* an: oxidefiof 'group 'of' elements represented by; be'ryllium'v andzirconium.

i'gnitorifc'omprising' infgredients'flof ibo'ronjp and carbon, 'and' an additionai'ingredient inclu; sion whereof 'gives tofl'the composition` a less n'e'g-` ative* and morepositive' temperature' coefiioient of:'resist'ance;v said4 'additional' ingre'dient' being 6.' :An ignitor' 'oomprisin'gf ingredients' of' boron' and carbon, and an additional ingredient inolu'- si'on whereo'f gives'fto*theificompfosition aless neg ativ'ej andi more* p'o'sitive'l temperature `coeflieiient' ofesistance, Jsaid? vadditional'=- ingrefdie'nt' being* 72 `1A1n ignitor conisi'stingof a 'mixti'ire-o'fE boroni ea-r'bidef-v1 boron; 'carbon antl'be'ryllial 8'; gnitor'fconsistingfio a mixt'ureoff boron' carbid'efboron1,'carbon*andl'zii'conia. i

9. An ignitor consisting of a mixture of ;boronh carbide; b'oron''l car-borny beryllia and'zii'conia';

' 10': Anffignitorf'consisting* of boronwcarbidej-f boronearbon' and a-meta-llie compound'iof'wliciithemetaP-is'selected from| the 'group off'beryllim* andlzirconiumfi I .RIGRQD yj REr'ERENoEs': 01m1): 'I The?followingzzrefereneesiare; off record? in the: file of this:.rn'ftei'it:.1,v UNrrsto'fivs'r'n'rEs'*V PATEN'rs:

2,1n6g85-'1s ,a s'mmemi Feb. 1.; 1988i 

